📚 IGCSE CIE Biology: Transcription Exam Focus | IGCSE CIE 生物：转录 考点精讲
Transcription is a fundamental process in molecular biology where the DNA sequence of a gene is copied into messenger RNA (mRNA). For IGCSE CIE Biology, understanding transcription is essential as it links genetics to protein synthesis. This article breaks down every key point you need to know, from the role of RNA polymerase to common exam pitfalls.
转录是分子生物学中的一个基本过程，基因的DNA序列被复制到信使RNA (mRNA) 中。对于IGCSE CIE生物，理解转录至关重要，因为它将遗传学与蛋白质合成联系起来。本文分解了你需要知道的每个关键点，从RNA聚合酶的作用到常见的考试陷阱。
1. What is Transcription? | 什么是转录？
Transcription is the first stage of protein synthesis. It involves copying a specific segment of DNA (a gene) into a complementary single-stranded mRNA molecule. This mRNA then carries the genetic code from the nucleus to the ribosomes in the cytoplasm for translation.
转录是蛋白质合成的第一阶段。它涉及将DNA的特定片段（一个基因）复制成互补的单链mRNA分子。然后，这个mRNA将遗传密码从细胞核带到细胞质中的核糖体进行翻译。
2. Location of Transcription | 转录的场所
In eukaryotic cells (such as animal and plant cells), transcription occurs inside the nucleus. The DNA is too large to leave the nucleus, so a smaller messenger molecule (mRNA) is made to transport the code. In prokaryotes (bacteria), transcription takes place in the cytoplasm because there is no nucleus.
在真核细胞（如动植物细胞）中，转录发生在细胞核内。DNA太大无法离开细胞核，因此会制造一个较小的信使分子（mRNA）来运送密码。在原核生物（细菌）中，转录发生在细胞质中，因为没有细胞核。
3. Key Players in Transcription | 转录的关键参与者
Several components are essential for transcription:
– DNA: The gene acts as a template.
– RNA polymerase: The enzyme that catalyses the synthesis of mRNA. It unwinds DNA and joins RNA nucleotides.
– Free RNA nucleotides: ATP, UTP, GTP, CTP (containing bases adenine, uracil, guanine, cytosine).
– Energy in the form of ATP is required to form phosphodiester bonds.
几个组成部分对转录至关重要：
– DNA：基因作为模板。
– RNA聚合酶：催化mRNA合成的酶。它能解开DNA并连接RNA核苷酸。
– 游离的RNA核苷酸：ATP、UTP、GTP、CTP（含有碱基腺嘌呤、尿嘧啶、鸟嘌呤、胞嘧啶）。
– 需要以ATP形式提供能量来形成磷酸二酯键。
4. The Template Strand | 模板链
Only one of the two DNA strands is used as a template for transcription. This is called the template strand (or antisense strand). The other strand is the coding strand (sense strand) and has the same sequence as the resulting mRNA (with thymine replaced by uracil). The enzyme reads the template strand in the 3′ to 5′ direction, building the mRNA in the 5′ to 3′ direction.
DNA的两条链中只有一条用作转录的模板，称为模板链（或反义链）。另一条链是编码链（正义链），其序列与生成的mRNA相同（但胸腺嘧啶被尿嘧啶取代）。酶以3’到5’的方向读取模板链，以5’到3’的方向合成mRNA。
5. Base Pairing Rules in Transcription | 转录中的碱基配对规则
During transcription, free RNA nucleotides pair with their complementary bases on the DNA template strand according to strict rules:
– Adenine (A) on DNA pairs with Uracil (U) on RNA.
– Thymine (T) on DNA pairs with Adenine (A) on RNA.
– Cytosine (C) pairs with Guanine (G).
– Guanine (G) pairs with Cytosine (C).
在转录过程中，游离的RNA核苷酸根据严格的规则与DNA模板链上的互补碱基配对：
– DNA上的腺嘌呤（A）与RNA上的尿嘧啶（U）配对。
– DNA上的胸腺嘧啶（T）与RNA上的腺嘌呤（A）配对。
– 胞嘧啶（C）与鸟嘌呤（G）配对。
– 鸟嘌呤（G）与胞嘧啶（C）配对。
DNA template: 3′ T A C G G A 5′
mRNA: 5′ A U G C C U 3′
The diagram shows a short segment; always write the mRNA from 5′ to 3′.
图中显示了一个短片段；书写mRNA时始终从5’写到3’。
6. Steps of Transcription: Initiation | 转录步骤：起始
Transcription begins when RNA polymerase attaches to a specific region of the gene called the promoter. The enzyme then unwinds and separates the two strands of the DNA double helix, exposing the template strand. No primer is needed for RNA polymerase to start synthesis—this is a key difference from DNA replication.
当RNA聚合酶附着到基因的一个特定区域（称为启动子）时，转录开始。然后，酶解开并分离DNA双螺旋的两条链，暴露出模板链。RNA聚合酶开始合成不需要引物——这是与DNA复制的一个关键区别。
7. Steps of Transcription: Elongation | 转录步骤：延伸
RNA polymerase moves along the template strand in the 3′ to 5′ direction. As it moves, it adds complementary RNA nucleotides one by one to the growing mRNA chain. The enzyme forms phosphodiester bonds between the nucleotides, creating the sugar-phosphate backbone of mRNA. The DNA double helix rewinds behind the enzyme.
RNA聚合酶沿模板链从3’到5’方向移动。移动过程中，它逐个添加互补的RNA核苷酸到正在延伸的mRNA链上。酶在核苷酸之间形成磷酸二酯键，形成mRNA的糖-磷酸骨架。DNA双螺旋在酶后方重新缠绕。
8. Steps of Transcription: Termination | 转录步骤：终止
Elongation continues until RNA polymerase reaches a terminator sequence on the DNA. At this point, the enzyme detaches from the DNA, the newly formed mRNA strand is released, and the DNA double helix fully rewinds. The completed mRNA transcript is now ready for the next stage.
延伸持续进行，直到RNA聚合酶到达DNA上的终止序列。此时，酶从DNA上脱离，新形成的mRNA链被释放，DNA双螺旋完全重新缠绕。完成的mRNA转录本现已准备好进入下一阶段。
9. The Product: Messenger RNA (mRNA) | 产物：信使RNA (mRNA)
The mRNA molecule is single-stranded and contains a sequence of bases complementary to the template DNA. This sequence is organized into codons (triplets of bases), each of which codes for a specific amino acid. In eukaryotes, the mRNA may undergo processing, such as the addition of a cap and tail, and splicing to remove introns, before leaving the nucleus. For IGCSE, the key point is that the mature mRNA exits through a nuclear pore to the cytoplasm.
mRNA分子是单链的，含有与模板DNA互补的碱基序列。这个序列被组织成密码子（三个碱基一组），每个密码子编码一个特定的氨基酸。在真核生物中，mRNA在离开细胞核前可能会经过加工，例如添加帽和尾，以及通过剪接去除内含子。对于IGCSE，关键点是成熟的mRNA通过核孔离开细胞核进入细胞质。
10. Comparison: Transcription vs. DNA Replication | 比较：转录与DNA复制
It is common for exams to ask you to compare transcription with DNA replication. The table below highlights the key differences:
考试中经常要求你比较转录和DNA复制。下表突出了关键区别：
| Feature 特征 |
Transcription 转录 |
DNA Replication DNA复制 |
|---|---|---|
| Purpose 目的 |
To produce mRNA for protein synthesis 产生用于蛋白质合成的mRNA |
To duplicate DNA before cell division 在细胞分裂前复制DNA |
| Enzyme 酶 |
RNA polymerase RNA聚合酶 |
DNA polymerase (+ helicase, ligase) DNA聚合酶（+解旋酶、连接酶） |
| Template 模板 |
One strand of a gene 一个基因的一条链 |
Both strands of the whole DNA 整个DNA的两条链 |
| Product 产物 |
Single-stranded mRNA 单链mRNA |
Two identical double-stranded DNA molecules 两个相同的双链DNA分子 |
| Base pairing 碱基配对 |
A-U, T-A, C-G, G-C A-U, T-A, C-G, G-C |
A-T, T-A, C-G, G-C A-T, T-A, C-G, G-C |
| Primer required? 需要引物？ |
No
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